Electrical devices, in particular, superconducting devices to be cooled to low temperatures, such as the winding of a magnetic coil or a generator, or a superconducting cable, require cooling systems ensuring the operability of the components to be cooled at the low operating temperature. Bath cooling, forced cooling or, in particular, indirect cooling can be used to cool these components.
Indirect cooling allows relatively compact, coolant-free cryostats to be built without coolant containers and frees the user from having to replenish the cryofluid. The required cooling effect can be achieved using a cryocooler, normally designed as a dual-stage cooler, which often works by the Gifford-McMahon principle. With such a cryocooler, the first stage may have a typical cooling capacity of 50 W at approximately 60K in the first stage and 1 W at 10K in the second stage.
Indirect cooling can be advantageously provided for superconducting magnet systems used for nuclear spin tomography. The corresponding cooling system must be designed so that as little as vibration as possible is transmitted to the magnet system when the refrigerating machine or a refrigerating machine component is thermally coupled to the superconducting magnet system. All conventional refrigerating machines have mechanically movable components causing considerable vibrations in the frequency range of 1 to a few tens of Hz. The pressure fluctuations of the working medium, typically helium at approximately 20 bar, can also contribute to the vibrations. If these vibrations act on the magnet system without being damped, undesirable eddy currents appear as the magnet system generating a basic magnetic field with an induction of 1 T, for example, is operated. These eddy currents not only increase the heat load on the refrigerating system, but also interfere with the imaging system of the nuclear spin tomography machine.
In order to solve the problems concerning transmission of vibrations, in a refrigerating system described in European Patent Application No. 0 260 036 for the He-cooled superconducting magnet of a nuclear spin tomography system, the magnet and a surrounding radiation shield are coupled to components of a refrigerating machine via flexible connecting elements made of a heat-conducting material. The damping characteristic requirements of such a coupling, also acting mechanically between a magnet and a refrigerating machine, are however, in general, considerably higher in the case of magnets for nuclear spin tomography.
U.S. Pat. No. 5,129,232 also describes a cooling system for the superconducting magnet of a nuclear spin tomography system with appropriate vibration-damping heat-conducting connecting elements between a refrigerating machine and a radiation shield/superconducting material. To further improve the vibration damping, the refrigerating machine is supported by the vacuum chamber that surrounds the superconducting winding via spring elements. These spring elements not only have to bear the weight of the refrigerating machine itself, but also the force of the external atmospheric pressure acting upon the ambient temperature section of the refrigerating machine. This pressure force is caused because the ambient temperature section is under the normal pressure surrounding the vacuum chamber of the superconducting winding, while the low-temperature section of the refrigerating machine is in an evacuated housing unit, which projects into the vacuum chamber of the superconducting magnet. Therefore, the spring elements are pressed together with a relatively great force and therefore must have a matching elastic force. The rigidity of the springs is, therefore, also high, so that vibration damping by the conventional spring elements is limited accordingly.